docs: add fractional_max_pool in nn.functional (#52557)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/51708
Pull Request resolved: https://github.com/pytorch/pytorch/pull/52557
Reviewed By: bdhirsh
Differential Revision: D26591388
Pulled By: jbschlosser
fbshipit-source-id: 42643864df92ea014e69a8ec5c29333735e98898
